Figure 9:
The
implementations of CAltAlt and
rely on many existing
technologies. The search engine is guided by heuristics extracted
from planning graphs.

The implementation of CAltAlt uses several off-the-shelf planning
software packages. Figure 9 shows a diagram of the system
architecture for CAltAlt and
. While CAltAlt extends the
name of
, it relies on a limited subset of the
implementation. The components of CAltAlt are the IPC parser for
PDDL 2.1 (slightly extended to allow uncertain initial conditions),
the HSP-r search engine [5], the IPP planning
graph [20], and the CUDD BDD package [7] to
implement the
labels. The custom parts of the implementation
include the action representation, belief state representation,
regression operator, and the heuristic calculation.

The implementation of
is very similar to CAltAlt aside from
the search engine, and state and action representation.
also
uses the IPP source code for planning graphs.
uses modified
LAO* [16] source code from Eric Hansen
to perform AO* search, and CUDD [7] to represent belief
states and actions. Even with deterministic actions it is possible
to obtain cycles from actions with observations because we are
planning in belief space.
constructs the search graph as a
directed acyclic graph by employing a cycle-checking algorithm. If
adding a hyper-edge to the search graph creates a cycle, then the
hyper-edge cannot represent an action in a strong plan and is hence
not added to the graph.